The most remarkable structural characteristic of the nervous system is the high degree of order with which its cells interconnect at sites of contact called synapses. Alterations in synapses underlie learning and memory; disruption of their development is likely to underlie numerous neurological and psychiatric diseases. The four participants in the program project propose to continue and enhance their collaborative, interdisciplinary studies on the intercellular interactions that regulate the formation and maturation of synapses. Their studies rely on a combination of recently developed methods for gene transfer and imaging which make it possible to gain fundamental insights into the cellular and molecular bases of these interactions. The focus for the next five years is on ways that activity-dependent and-independent regulatory programs cooperate to generate patterns of synaptic connectivity in mice. Two of the four investigators, Lichtman and Sanes, will combine their expertise in imaging and transgenic technology, respectively, to study the skeletal neuromuscular junction, whose relative simplicity and accessibility have made it the best understood of all synapses, In one set of studies, Sanes will use transgenic methods to block the electrical activity of some or all motor axons or muscle fibers during embryogenesis or postnatal life. Lichtman will image synapses in these lines over minutes to weeks, to learn how activity affects synaptic maturation and competition. Our studies will use genetically engineered mice in which only a few motor axons that supply a muscle are indelibly marked with a fluorescent tag; these will be imaged to elucidate position- and activity-dependent aspects of motor unit maturation. The other two investigators, Craig and Wong, will use many of the same lines and markers but extend the investigation to synapses in the brain. Wong will ask how presynaptic cells in the retina, and the activity they generate, shape the dendritic arbors of their postsynaptic cells in the retina, and the activity they generate., shape the dendritic arbors of their postsynaptic partners. Craig will use dissociated cell cultures to ask detailed developmental questions about how central (hippocampal) synapses. Her work will illuminate mechanisms that can not be studied in muscle, such as how individual neurons segregate synapses of different types to distinct domains.